This invention relates to protective circuitry for semiconductor switching devices, and more particularly to a protective circuit for a switching transistor which connects an inductive load to a voltage supply.
A protective circuit which connects an inductive load to a voltage supply, and which is shunted by a diode in series with a capacitor, is described in German reference DE-AS No. 19 56 145. In that reference, each switching transistor is connected in a shunt configuration with a series combination of a diode and a capacitor, for protecting inductively loaded switching transistors in electronic push-pull converters. The capacitor serves to protect the electronic switch from overvoltages which occur when the inductive load is disconnected. The diode is shunted by a transistor which is switched into a conductive state during the time that the transistor to be protected is in a nonconductive state. During such time as the shunt transistor is in a nonconductive state, the charge on the capacitor which is accummulated as a result of the switching transistor being switched off is discharged back into the voltage supply by means of a primary winding of a converter transformer. Thus, a portion of the capacitor discharge energy is conducted to the voltage supply, and a further portion is delivered to the output circuit of the converter.
In the above-described prior art circuit arrangement, the capacitor can be discharged only to the voltage level of the input voltage. During the time that the shunt transistor is in a nonconductive state, the voltage thereacross rises to at least the value of the input voltage. Thus, when the transistor becomes conductive once again, it experiences considerable stress because it must conduct a considerable current at a relatively high voltage, thereby requiring a relatively large power dissipation. Such a disadvantageous power dissipation requirement results from the fact that the capacitor discharge energy, in this prior art circuit arrangement, can be conducted only to either the voltage supply, or to the output circuit of the converter. No other utilization of the dischage energy is possible with this circuit.
It is, therefore, an object of this invention to provide a protective circuit for a switching transistor which connects an inductive load to a voltage supply, whereby the discharge energy of a capacitor can be used in any manner desired.